There are a number of industrial applications where it is desired to have a fluid which is highly viscous at one time and of relatively low viscosity at a later time; for example, where it is desired that the fluid suspend particulate matter while it is in transit and then deposit the particulates or allow them to settle at a given place or time. The present invention is concerned with such fluids.
One especially important application of such fluids is in the practice of fracturing subterranean formations. A (hydraulic) fracturing fluid having granular insoluble particulate material suspended or slurried therein is injected down a well penetrating the formation and forced back into the formation. This causes the formation to crack or fracture thereby improving the flow of oil, gas, water, etc., between the formation and the well. Fracturing is customarily conducted by employing oil, water, (usually thickened) or an emulsion thereof which has suspended therein particles (proppants) which are substantially insoluble in the liquid and the fluids of the formation. The particles are thereby forced into the fractures created, thus propping open the fractures when the fracturing pressures are subsequently released.
To maintain a proppant suspended in the fluid until it is lodged in a fracture presents problems since the propping agent tends to settle out. This sometimes occurs prematurely resulting in what is known as a sand-out or a screen-out. In efforts to avoid this premature deposition of proppants, the suspending ability of fracturing fluids are improved by thickening them with a suspending agent (a viscosity increasing agent). Although this aids in maintaining the propping agent in suspension until it is lodged in the fraction, it tends to prevent the removal of the fracturing liquid from the fractures.
The use of additives, also called "breakers", to reduce the viscosity of various polymeric thickening agents is known. In U.S. Pat. No. 2,483,936 the viscosity of an aqueous solution of guar gum is reduced by dispersing in the solution sodium and hydrogen peroxide. In U.S. Pat. No. 3,024,191 the viscosity of a carboxymethylcellulose is reduced with a perborate. In U.S. Pat. No. 3,167,510 the viscosity of aqueous solutions thickened with guar gum are decreased by employing a persulfate. In U.S. Pat. No. 3,442,803 a polymer a acrylamide and methylenebisacrylamide is broken by adding a water soluble persulfate or perborate thereto.
It has now been discovered that the viscosity of an aqueous solution containing a polymeric thickener, e.g. xanthan gum, can be controllably broken by the addition thereto of an alkali metal hypochlorite and cyanuric acid.